1. Field of the invention
The invention relates to a non-destructive process for characterizing the surface condition of a part in order to check its roughness and reveal any defects. The process may be applied in particular to the examination of shot-blasted surfaces and is especially suitable for inspection of areas with difficult access.
Shot blasting processes consist of bombarding the surface of a part with steel, glass or ceramic balls. These processes are used particularly in applications seeking to obtain surface compression prestressing in order to improve the operating performance of the parts by increasing the fatigue resistance or the corrosion resistance under stress. As the complexity of the shape of the part to be treated increases, it becomes more difficult to obtain the required degree of application of the shot to the part. A good quality assurance of parts is required more and more, and it is therefore necessary to ensure that the degree of shot blasting is sufficient and that the shot blasting extends without interruption over the whole of the treated surface.
2. Summary of the Prior Art
There are a variety of methods enabling the surface condition of a part to be checked. In particular it is known to check the surface condition of parts by the inspection of imprints. Each imprint is obtained by placing on the surface to be examined a sheet of plastic film, for example of cellulose acetate, soaked in acetone so as to soften it in order for it to adopt faithfully the shape of the part. After hardening, the imprinted sheet is removed and may be viewed using a binocular magnifier, an optical microscope or a scanning electron microscope. This technique is currently used for examining shot-blasted, sanded, ground or machined surfaces, as it makes it possible at the same time to validate parts, store the surface conditions through the imprints, and carry out topographic appraisals. However, the technique is difficult to carry out and is inapplicable in areas which are difficult to access. Moreover, precautions have to be taken to prevent the imprints exhibiting serious defects, such as air inclusions. Furthermore, the technique is difficult to apply when the ambient temperature is high because of the rapid evaporation of acetone.
It is also known to check the surface condition of a part by taking dimensional readings which enable a measurement of roughness to be obtained. In particular, in the field of metrology dimensional readings are taken from imprints made using self-polymerizing methyl methacrylate based fluid resins belonging to the acrylic resin family. However, this type of resin cannot be used for checking the condition of surfaces in inaccessible areas as it is too fluid and very adherent after hardening. Moreover, the accuracy of detail obtained with this type of resin is insufficient and, generally, it contains numerous air inclusions.
The surface condition resulting from shot-blasting may also be checked from technological test pieces representative of the parts subjected to the blast of shot. However, making test pieces is a constraint in terms of cost and does not supply an accurate indication of the real surface condition of the parts.